This invention relates generally to methods and apparatus for CT imaging and other radiation imaging systems, and more particularly to detecting and correcting data for tube-spit events.
In at least some computed tomography (CT) imaging system configurations, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the xe2x80x9cimaging planexe2x80x9d. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal spot. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator adjacent the collimator, and photodetectors adjacent the scintillator.
The term xe2x80x9ctube-spitxe2x80x9d refers to temporary electrical short-circuit that sometimes occurs inside an x-ray tube. Typically, upon the occurrence of tube-spit, the supply of power to the x-ray tube is temporarily interrupted to prevent arcing. Power is restored to the tube after a time interval of, for example, about one millisecond. During tube-spit recovery, no x-ray photon is emitted from the x-ray tube. As a result, the detector measurements taken during the recovery are invalid.
Generally, tube-spit recover time should be shorter than the data acquisition sampling interval to avoid image artifacts. As data sampling rates increase, a recovery time of about one millisecond is too long to avoid artifact generation.
In one aspect, the present invention relates to tube-spit detection and correction that can be performed utilizing software processing rather than increasing demands on hardware by reducing the time interval between switching power to the x-ray tube off and on. In one exemplary embodiment, and once a sample of projection data has been collected, the projection data is preprocessed in accordance with known preprocessing algorithms including applying a detector primary speed correction to the projection data. Such primary speed correction removes contamination of previous signal sample from the current sample.
After preprocessing, tube-spit detection is performed on the sample. Such detection can be performed using many different methods. Generally, the objective is to determine whether the x-ray source experienced a drop in power. Specifically, upon the occurrence of a tube-spit event, signals on the entire detector should experience a significant drop since the x-ray tube output is reduced to nearly zero. Therefore, a tube-spit condition can be detected by determining whether the x-ray tube output was reduced to nearly zero for a period of time.
If a tube-spit event is not detected, then processing proceeds without tube spit correction. If a tube-spit event is detected, then a tube spit correction is performed. In addition, if a tube-spit event is detected, power is still interrupted to the x-ray tube. The tube-spit correction described herein, however, eliminates a need to increase demands on hardware by reducing the time interval between interrupting power to the x-ray tube as compared to the time interval described above, i.e., even with increased sampling rates, the time interval for interrupting power to the x-ray tube need not be reduced. Generally, the objective of tube-spit correction is to remove image artifacts due to the occurrence of a tube-spit event. Many different methods can be utilized to perform tube-spit correction.